Electron equipment generally needs a cover with a shield material capable of shutting off electromagnetic radiation. The cover is required for preventing electromagnetic irradiation into or out of electronic apparatus and equipment.
Effective as a shield against electromagnetic radiation (hereinafter referred to as "EMR shield") are coatings formed from electrically-conductive materials. EMR shields have been produced by application of electrically-conductive paint, metal spraying, metal sputtering, adhesion of metallic foil, plating or the like to deposit a metal layer on a nonconductor. Since the effect of EMR shield has correlation with electrical conductivity, metals of high electrical conductivity such as gold, silver, copper and the like are suitable as coating materials. Generally copper, being inexpensive, is used as such material. However, copper is prone to oxidation and, on oxidation, reduces the electrical conductivity, impairing the effect of EMR shield. To diminish this defect, copper is usually coated with Ni, Co or like oxidation-resistant metals for protection. For example, U.S. Pat. No. 4,514,586 discloses an EMR shield material prepared by electrolessly plating the surface of a nonconductor with copper and then the copper layer with metals comprising nickel, cobalt, gold and alloy of these metals (these metals will be hereinafter represented by "Ni"). The disclosed method applies a catalyst layer of Pd, Pt, Au or the like to the copper layer prior to electroless plating of copper layer with Ni and like metals, since Ni and like metals are unable to deposit directly on the copper layer. However, the deposition of catalyst layer poses the problem of introducing Pd, Pt, Au or like catalyst component into the electroless plating solution containing nickel and other metals to result in the contamination and decomposition of the solution. Further the method entails difficulty in controlling the thickness of catalyst layer of Pd, Pt, Au or the like. More specifically, when a nonconductor is immersed in the catalyst solution for a prolonged period of time to provide the desired deposition of Ni or the like, the thickness of catalyst layer is excessively increased, thereby deteriorating the adhesion between the metallic deposits.